Fissionable nuclear fuel materials for the nuclear reactors comprise one of two principal chemical forms. One distinct type consists of fissionable elements such as uranium, plutonium or thorium, and mixtures thereof, in metallic, non-oxide form. Specifically this category comprises uranium, plutonium or thorium metal, and mixtures or alloys of such metals.
The other principal type of nuclear reactor fuel consists of ceramic or non-metallic oxides of fissionable and/or fertile elements comprising oxides of uranium, plutonium or thorium, and mixtures of such oxides. Uranium oxides, especially uranium dioxide, has become a standard form of fissionable fuel for commercial nuclear power plants used to generate electrical power. However, minor amounts of other fissionable materials, such as oxides of plutonium and thorium, and/or depletable neutron absorbers, sometimes referred to in the art as "burnable poison", such as gadolinium oxides, are commonly included in the fuel with the uranium dioxide.
In conventional nuclear reactors, atoms of the fissionable material comprising uranium and plutonium isotopes absorb neutrons into their nuclei and undergo a nuclear disintegrating or splitting. This fission reaction produces on the average of two products of lower atomic weight and greater kinetic energy, and typically two or three neutrons, also of high energy.
The fission produced neutrons diffuse through the reactor core containing fissionable fuel and are either utilized or lost in several distinct competing mechanisms. Some neutrons undergo nonfission or radiative capture in the fuel material. Other neutrons undergo fission capture within the fissionable fuel and thereby produce additional fission neutrons, the so-called chain reaction. Namely, fast neutrons are captured in the uranium 235 and 238, while thermal neutrons are captured in uranium 235. Still other neutrons undergo parasitic capture in the various extraneous or nonfissionable compositions of the fuel core and adjoining components such as the moderator, coolant, various structural materials, fission products produced within the fuel, as well as any neutron absorbing reaction control materials applied to regulate the fission rate.
The balance between the fission production of neutrons and the various competing mechanisms for neutron consumption determine whether the fission reaction is self-sustaining, decreasing, or increasing. When the fission reaction is self-sustaining, the neutron multiplication factor equals 1.0, the neutron population remains constant, and on average there is one neutron remaining from each fission event which induces a subsequent fission of an atom.
Heat produced by the fission reactions is thereby continuous and maintained as long as sufficient fissionable material is present in the fuel core to override the effects of fission products formed by the reaction, some of which have a high capacity for absorbing neutrons. The heat produced by the fission reactions is removed by a coolant such as water, circulating through the reactor core in contract with the containers of fuel and conveyed on to means for its utilization, such as the generation of electrical power.
The neutron population, and in turn the heat or power produced, of a nuclear reactor, depends on the extent to which neutrons are consumed or wasted by capture in nonfissionable, neutron absorbing material. Neutron consumption of this nature is regulated by governing the relative amount and capacity of neutron absorbing control materials imposed into the core of fissionable fuel material undergoing fission reactions.
In any case, the fission reactivity of the fuel progressively decreases with time in service due in large part to fission-product accumulation within the fuel. This progressive depletion of fission reactively is typically compensated for by withdrawal of the neutron absorbing control rods whereby the neutron population available to perpetuate fission is regulated to maintain a continuing level of reactivity.
To achieve greater efficiency and economy in the operation of nuclear reactor plants efforts have been made to extend the service life of the fuel between refueling cycles. One common measure for prolonging fuel performances has been to utilize a fuel having excessive reactivity in combination with a depletable neutron absorbent, frequently referred to in the nuclear reactor art as a "burnable poison." Thus, the initial excessive reactivity of the fuel is tempered by the introduction of a depletable neutron absorbent, or "burnable poison", such as gadolinium oxide which progressively expends its capacity for neutron due to their absorption. Thus, prolonged fuel service is provided with the high reactivity fuel while the initial excessive reactivity of the fuel is negated by the removal of fission producing neutrons with a depletable neutron absorbent which serves to level or stabilize the fuel reactivity rate over its services life. The depletable absorber is utilized so as to absorb neutrons at a decreasing rate approximately commensurate with the diminishing reactivity of the fuel whereby a substantially constant rate of reactivity is maintained through the cycle.
The practice and means for this fuel performance extending measure are disclosed in detail in U.S. Letters Pat. No. 3,799,839, issued March 26, 1974. Depletable neutron absorbent agents and their relevant properties are also disclosed in an article entitled "Nuclear Theory And Calculations For Burnable Poison Design" by W. A. Northrop, pages 123-150, published in Neutron Absorber Materials For Reactor Control, Untied States Atomic Energy Commission.
The disclosure and contents of the foregoing cited background publication are incorporated herein by reference.